Lotus leaves famously exhibit superhydrophobicity — the ability of a surface to strongly repel water. Sometimes called the 'lotus effect', plants exploit this property for self-cleaning purposes: tiny bits of dirt are captured in a water droplet as it rolls over a leaf. It is now well established that (super)hydrophobicity is caused by microscale surface roughness, and nature's skill has been mimicked in various coating technologies, such as liquid-repellent tablecloths.
The phenomenon is not restricted to water, a polar liquid: 'amphiphobicity' is the general term for both polar and nonpolar liquid repellence. The usual way of thinking about superamphiphobicity is in terms of a liquid drop wetting, or rather not quite wetting, a flat or slightly curved solid surface. But Ming Ye and colleagues take the opposite approach and consider a spherical particle coated with a superamphiphobic layer floating on a liquid surface (Phys. Rev. Lett. 112, 016101; 2014). They wondered how small superamphiphobically coated particles can be — in other words, whether there are limits to the curvature of such coatings.
Ye et al. formed amphiphobic layers from nanoparticles of candle soot (with radii around 60 nm), assembled into structures resembling bead strands, and applied them as a coating to glass microspheres with radii of about 20 μm. The superamphiphobic effect has its origin in the nanoporous structure of the coating. When the coated microspheres are brought in contact with liquid/air interfaces, the liquid can only penetrate a little way into the applied layer; the many 'air cushions' block the liquid and result in a large macroscopic contact angle.
With one end of a coated microsphere attached to a piezo-unit (capable of converting displacements into forces and vice versa), Ye and colleagues were able to quantitatively investigate the adhesion of a coated particle to a liquid surface, with nanonewton precision, and obtain further qualitative understanding of superamphiphobicity. In particular, they have arrived at an expression for the minimal radius of the coated surface that results in superamphiphobicity, involving the surface tension of the liquid, the radius of the nanoparticles and the typical spacing between strands of them. Above the critical curvature — which is smaller for oil than for water — the capillary pressure exerted by the liquid on the superamphiphobic layer is larger than the depinning pressure required to force the trapped air out of it.
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Verberck, B. Bend setters. Nature Phys 10, 95 (2014). https://doi.org/10.1038/nphys2889